Optical character scanners are devices for digitizing documents by converting character images on the document into digital data form for subsequent use in analyzing, storing and reprinting a copy of the original so as to eliminate hours of tedious typing. Optical scanners are currently used in a variety of applications to perform steps which were previously executed manually.
Optical scanners are well known in the art. In such systems, a light source is typically directed toward the document while an array of photodetectors scans the document. During scanning, the array detects light reflected from the images on the document. Digital data is generated corresponding to the detected light, and thus, to the image on the document from which data is being generated relative to a reference location.
The use of optical scanners for reproducing typed form documents and the like has been somewhat limited by the difficulty realized in calibrating the imaging system to differentiate between characters comprising the text to be reproduced via the optical scanner and aberrant marks or poorly formed characters which cannot be recognized by the scanning system. A poorly calibrated system results in a failure of the scanning system to retrieve all of the data required by the operator.
The calibration of optical scanners is typically effected by a manual process wherein the scanner user is required to optimize various threshold parameters by visual comparisons of the resulting images in order to ensure that all of the text is converted digitally without causing the scanner to detect "ghost" images or otherwise "misread" the image.
Various methods have been proposed in the art for calibrating optical scanners. For example, U.S. Pat. No. 4,408,231 discloses a method for calibrating a photosensitive scanning array wherein a linear image sensor and the video channel associated with the sensor are calibrated to maximize the video signal available. The useable video signal is maximized by calibrating the sensor relative to the range of the analog-to-digital converter. The sensor is calibrated by adjusting its exposure to illumination. The adjustment may be made by adjusting the illumination source or by changing the sampling interval. The gain is adjusted by using a variable gain amplifier. A microprocessor monitors the digitized video signal and controls the exposure and gain to calibrate the sensor and channel.
U.S. Pat. No. 4,605,970 discloses a method and apparatus for calibrating an optical document digitizer in which the calibration mechanism is mounted to a rotatable drum of the digitizer for such purpose. The apparatus comprises an elongated strip having an optical pattern of contrasting light and dark areas on the surface thereof, the pattern including plural discrete blocks, each of the blocks having first and second orthogonal edges wherein the first edges are positioned along a scanning reference line.
A need in the art exists for a relatively inexpensive and practical method suitable for reliably calibrating an optical scanner which does not utilize the cumbersome and expensive apparatus or the time consuming subjective manual approach of prior art systems.